Beam systems, such as electron beam systems, ion beam systems, laser beam systems, cluster beam systems, and neutral particle beam systems, are used to create features on a surface by etching or depositing material. Focused beams are used to remove material from a sample and to deposit material onto the sample. Material can be removed by sputtering, in which the momentum of the particles in the beam physically knock atom or molecules from the sample surface.
A particle or laser beam can be used to induce a chemical reaction. In some cases, the beam induces decomposition of a precursor gas. The precursor gas is preferably stable so that it does not react with the work piece away from the beam impact area. The resolution of the deposit or etching is determined by the beam diameter and region of interaction between the beam and the work piece. “Resolution” is used herein to refer to the smallest feature size that a process can produce.
In beam-induced deposition, the decomposition products include a non-volatile product that remains on the work piece and a volatile product that is eventually removed by the vacuum pump. For example, a gaseous organometallic compound, such as tungsten hexacarbonyl, may be provided near the sample and is adsorbed onto the surface. The beam decomposes the tungsten hexacarbonyl to leave tungsten on the work piece at the points of beam impact.
In beam-induced etching, the precursor gas forms a volatile byproduct with the work piece material, which is eventually removed by the vacuum pump. For example, iodine can be used as a precursor gas to etch silicon, the iodine forming volatile compounds with the silicon in the presence of the beam. Many deposition precursors and etch precursors are known in the art. In some cases, such as a beam of carbon 60 particles, material in the beam are directly deposited onto the surface without disassociating a precursor.
Different types of beams provide different amounts of energy and have different spot sizes at the sample. Higher energies typically correspond to higher etch or deposition rates, but lower resolution. While a charged particle beam can be focused into a much smaller spot than a laser beam, the size of the beam spot on the work piece is typically related to the current in the beam. Beam current is usually determined by the size of an aperture in the beam path. A smaller aperture blocks more of the off-axis particles, which typically do not focus as well as the particles near the axis. Blocking the off-axis particles reduces the beam current. Also, reducing the beam current reduces the tendency of the beam to spread out due to the repulsive force of the charged particles in the beam. Changing the aperture typically requires physically moving a new aperture into the beam path and centering it, which takes some time. In some systems, current may also be controlled by controlling source settings, such as the plasma density or extraction voltage.
A typical focused ion beam system using a liquid metal ion source can produce a current of between about 1 pA to about 100 nA with a spot size of between about 3 nm and 3 mm. A plasma ion focused ion beam source can produce currents between about 1.5 pA to about 1.5 μA with a spot size of between about 4 nm and about 5 mm. Electron beam currents are typically between about 0.5 pA to about 0.5 μA with a spot size from less than a nanometer to about 3 nm. A small, high resolution beam typically has a low current, which produces a low etch or deposition rate. For example, the rate of focused electron beam-induced deposition is typically around about 5×10−4 μm3·nC−1. An ion beam can typically deposit a film using beam-induced deposition at a rate of up to about one micron per minute.
Three-dimensional structures can be formed by multiple scans of the beam, with additional material being etched or deposited on each scan. Each point to which a beam is addressed is referred to as a “dwell point.” The period during which a beam remains at a point is referred to as the “dwell period.” The total amount of particles or energy provided to a point is referred to as the “dose,” and can refer to the dose during a single dwell period, or to multiple dwell periods. A scan may refer to a raster pattern in which the beam scans over a processing area in a regular pattern, such as a rectangle, or may refer to a scan in which the beam is directed toward individual points in an irregular pattern.
A pattern to be fabricated may be represented by a “bitmap,” which shows the dwell points to which the beam is to be directed. The beam can be deflected rapidly across points on the work piece that are not to be addressed, or the beam can be blanked between dwell points. To form a pattern of deposited or etched material, current systems allows a user to specify which points on the X,Y plane to expose and a dwell period specified for each point. When the machine operator desires to form a pattern having both large and small features, the operator needs to select a beam that provides sufficient resolution to produce the finer features, which results in an excessive time for forming larger features that do not require fine resolution.